Disruption of services: Services such as Visual Voicemail, YouTube, Weather, and Stocks have been disrupted or no longer work on the device. Additionally, third-party apps that use the Apple Push Notification Service have had difficulty receiving notifications or received notifications that were intended for a different hacked device. Other push-based services such as MobileMe and Exchange have experienced problems synchronizing data with their respective servers.

Compromised security: Security compromises have been introduced by these modifications that could allow hackers to steal personal information, damage the device, attack the wireless network, or introduce malware or viruses.

Shortened battery life: The hacked software has caused an accelerated battery drain that shortens the operation of an iPhone, iPad, or iPod touch on a single battery charge.

Inability to apply future software updates: Some unauthorized modifications have caused damage to the iOS that is not repairable. This can result in the hacked iPhone, iPad, or iPod touch becoming permanently inoperable when a future Apple-supplied iOS update is installed.

The question has been closed for being "Solved" by
spockers,
23 Nov '13, 02:59

I like to think that jailbreaking isn't harmful by itself, but it does gives you the rope to hang yourself.

If you install a bad MobileSubstrate addon, it can certainly cause instability, but the only case I know of MobileSubstrate itself causing crashes occurred when Skype used some very dodgy tricks to obfuscate itself, and that's been fixed.

Some packages might shorten your battery life, but the default setup shouldn't affect it at all. (A long time ago, SSH affected battery life, but now it uses launchd and should have zero effect when it's not running.)

Although a jailbreak theoretically weakens some of iOS's built in security measures, most of them are still in place, and I've never heard of a non-trivial iOS exploit ever being used maliciously. However, a lot of people install SSH and then just leave the default password - alpine. In that case, anyone who can open a TCP connection to your device can trivially pwn it.

The last time an update has ever bricked someone's device was with iPhone OS 1.x or something; generally, DFU mode can always save you no matter how badly you screw things up.

Completely perceive what your pose on this issue. Although I would disagree on some of the finer specifics, I believe you did an superior job explaining it. Certain beats having to research it by myself. thank you.

the biggest "negative effect" / problem for most would be default SSH passwords. people install SSH and just don't bother to change the password, not exactly realizing how easily their device could be compromised.

Shortened battery life: The hacked software has caused an accelerated battery drain that shortens the operation of an iPhone, iPad, or iPod touch on a single battery charge.

This is indeed true, but only in the same way that watching video, or playing intensive games, can result in a shortened battery life. Because jailbreaking allows users to do so much more with their iOS device, the device simply gets used more and so has a reduced battery life. If a user jailbreaks their device but installs no jb apps, then their battery life will remain the same.

With jailbreaking you don't void warranty. Just restore to stock firmware and you're back like non-jailbroken users. The only exception is installing baseband 6.15 on an iPhone, which cannot be reversed.